could you give me your expertise for following case:It seems that someone uses parts of Joomleague-Code and graphics, e.g. placeholder for clubs in tables, without a reference to the author/creator of all these things. Isn't it legal?

According to GPL a note for the original copyright has to be clearly stated! If it is a source code file the note has to be placed within the code! Removing notes of a copyright/author in an existing src code file which gets redistributed in an own component (like a fork) is a GPL violation!